Power transmission lines expand their length due to thermal loading. A resulting sag of overhead lines is undesirable. As a result, the length or a length change of the power transmission lines may be monitored. An increase in length of overhead power transmission lines may lead to sag. Line sag of high voltage lines is important, as it may lead to line faults due to flashovers to close objects such as trees.
In Wide Area Monitoring (WAM) systems for AC power transmission grids, Phasor Measurement Units (PMUs) perform measurement of voltage and current phasors. U.S. 2007/0038396 discloses a method in which voltage and current phasors measured at two ends of a power transmission line are used to determine a line impedance, and specifically a line resistance. The resistance is a function of line temperature, so that the method provides a measurement of the average line temperature, which in turn affects the line length and its sag. The method requires synchronization of the PMUs.
Further, there are direct powerline sag measurement methods based on various principles, such as optical cameras, mechanical tension or inclination measurement (see, e.g., U.S. Pat. No. 5,235,861, U.S. Pat. No. 6,523,424, U.S. Pat. No. 6,205,867), thermal measurements using fibres (see, e.g., U.S. Pat. No. 6,776,522) or infrared cameras. These methods, however, only perform local measurements.